1. Field of the Invention
The present invention relates to a thermal head used for a thermal printer, and particularly to a thermal head comprising a heat insulating layer formed by vapor deposition such as sputtering in order to improve the printing life, wherein the surface of the heat insulating layer is polished.
2. Description of the Related Art
A conventional thermal head comprises a heat insulating layer formed on a heat radiating substrate by vapor deposition such as sputtering or the like, and a plurality of heating elements linearly arranged on the heat insulating layer so that current is selectively passed through the heating elements to record a dot image using heat sensitive recording paper or a heat transfer ribbon.
In the example shown in FIG. 5, the thermal head comprises a heat insulating layer 12 formed, by sputtering, in a thickness of about 20 .mu.m on a substrate 11 of silicon having excellent radiating property, composed of silicon, transition metals and oxygen, and having excellent heat resistance. In the process for depositing the heat insulating layer 12, the sputtering pressure is as high as about 1.0 Pa in order to intentionally form columnar crystals and deposit the layer with a low density, to obtain the heat insulating layer 12 having excellent heat insulating property.
However, since the heat insulating layer 12 comprises columnar crystals, the surface thereof exhibits a rough state having initial irregularity 12a. Also abnormal projections 12b occur due to contaminant particles peculiar to the vapor deposition process. The contaminant particles represent particles produced by peeling of a film deposited in the vacuum container of a vapor deposition apparatus such as a sputtering apparatus or the like, and floating as particles having a size of 0.1 to several micrometers in the vacuum container. In film deposition, the contaminant particles adhere to the substrate surface to produce projections in the film formed on the substrate surface with the contaminant particles as nuclei.
FIG. 6 is a drawing showing a three-dimensional image of the surface of the heat insulating layer 12, which was output by using an atomic force microprobe AFM. As the result of measurement of the surface roughness (Rz), Rz=45 nm.
Although not shown in the drawings, on the heat insulating layer 12 are formed a heating resistor, and a common electrode and individual electrodes for passing a current through the heating resistor. A protecting layer is further coated for protecting the heating resistor and each of the electrodes from oxidation and abrasion to form a thermal head.
The heat insulating layer 12 comprising columnar crystals and formed by sputtering as described above has a high degree of defects such as variations in the dot resistance value, disconnection and short-circuit of the electrode pattern, apparent foreign materials, etc. due to the initial irregularity 12a and the abnormal projections 12b formed with the contaminant particles peculiar to the vapor deposition process as nuclei, and thus has the problem of deteriorating the product quality and production yield.
Therefore, the applicant already proposed that the initial irregularity 12a peculiar to the columnar crystals on the surface of the heat insulating layer 12, and the macroscopic abnormal projections 12b formed with the contaminant particles as nuclei are removed by chemical polishing to form substantially a mirror surface, thereby solving the problem of deteriorating product quality and production yield. FIG. 3 is a schematic drawing showing the surface of the heat insulating layer after chemical polishing. In FIG. 3, reference numerals 12a and 12b denote portions corresponding to the initial irregularity and abnormal projections, respectively, shown in FIG. 5.
FIG. 4 is a drawing showing a three-dimensional image of the surface of the heat insulating layer 12 after polishing, which was output by the atomic force microprobe AFM. In this case, the surface is a smooth surface having less irregularity and a surface roughness Rz=4.5 nm.
However, as a result of a printing durability test of the thermal head comprising the heating elements formed on the heat insulating layer 12 chemically polished to substantially a mirror surface, the actual printing life was about 20,000,000 to 50,000,000 characters. This was due to a trouble mode in which, in printing runs, the protecting layer is cracked due to deterioration in adhesion of the films in the upper and lower interfaces of the heating resistor formed on the heat insulating layer 12, thereby causing dot defects due to oxidation of the heating resistor. This was caused by the excessive flatness of the surface of the heat insulating layer 12 as a base, and it was thus found that the surface must be modified to increase the adhesion.
In recent years, mass production of thermal heads comprising a silicon substrate with excellent heat responsiveness in order to improve printing quality has been made, and the contact pressure between a thermal head and a printing medium (a heat transfer ribbon or heat sensitive paper) has been increased in order to improve printing quality for plain paper. Therefore, in a printing operation, high shearing stress is applied to the thermal head, as compared with previous thermal heads. The shearing stress causes peeling due to fatigue failure in the upper and lower interfaces of the heating resistor, thereby interfering with an increase in the printing life.